This invention relates to electromagnetic clutches and, in particular, to a method for producing a magnetic rotatable member in such an electromagnetic clutch.
An electromagnetic clutch which is mounted on a device having a drive shaft to selectively drive the device by an external power source has been known and used in the prior art, as disclosed in, for example, U.S. Pat. Nos. 3,044,594, 3,082,933, and others.
Such an electromagnetic clutch comprises a magnetic pulley rotatably mounted on a housing of the device and being rotated by an external power source through a belt, and an armature plate fixed onto the drive shaft of the device. An electromagnetic coil or an electromagnet associates with the pulley to magnetically attract the armature plate to the pulley at a time when being energized so that the rotation of the pulley is transmitted to the drive shaft. Furthermore, the pulley is provided with an annular hollow portion in which the electromagnetic coil should be disposed and with belt receiving grooves. Therefore, since the pulley is complex in the shape, it takes a long time to form the pulley. That is, the pulley is formed through a long forging process and is finished to accurate dimensions through a machining step. In the machining step, a large amount of material is removed. Therefore, in the known method for producing the pulley, much material is wasted. Thus, the cost of the electromagnetic clutch is increased.
In order to reduce the weight of the pulley, it has been made to constitute the pulley by two pieces of an annular body of magnetic materials such as steel and a belt receiving groove member of aluminum or aluminum alloy fixed on the outer surface of the annular body. The resultant pulley is light in the weight. However, the surface of the magnetic portion opposing the armature plate is reduced so that the rotation transmitting force may be reduced. In order to increase the surface of the magnetic portion opposing to the armature plate, it is used to form an annular rim radially projecting on the outer surface of the annular body at an axial end thereof opposing to the armature plate. Therefore, the magnetic annular body is relatively complex in the shape. Accordingly, if it is formed by forging following machining, the wasted material is still much. And if it is press-formed from an annular plate, a large amount of material must be removed by machining to form the radially projecting annular rim.
Furthermore, either a single part of pulley of magnetic material or an annular body as a part of the pulley must be provided with a stopper for a bearing on which the pulley should be mounted so that the shape of them is further complex.